This is a tutorial on how to add calendar permissions in Office 365 for your users via Powershell. You can add permissions onto a specific mailbox, or you can add it onto a security group.
By default, Exchange (and Office 365) users can’t view messages or calendar items of other users. The only permission that is provided to all users by default is the ability to view free/busy information in the calendar of other users (AvailabilityOnly role).
Office 365 Calendar Permissions
The first is step is to launch Windows PowerShell. We recommend running it as Administrator.
Run the following command to login to 365 via Powershell and login with your Office 365 admin credentials:
In some cases, you need to grant Reviewer permissions on a calendar folder in all mailboxes to all users in your Exchange organization. You can make this bulk permission change using simple PowerShell script. To change Default calendar permission for all mailbox in mailbox database to Reviewer: